An optical isolator provides isolation between a control circuit and a switched circuit. A typical optical isolator has a light emitting diode (LED) electronically coupled to the control circuit. Often the LED is connected in series with the control circuit, and it thus uses current derived from the control signal to illuminate the LED rather than from the circuit's power supplies. The LED is optically coupled across an electronically isolating gap to a photovoltaic device (such as a photodiode). The photodiode is in turn connected to switching circuitry that is compatible with the power and current requirements of the switched circuit. In a typical optical isolator circuit, a comparator is used to provide the power and current requirements for the switched circuit. Light from the LED creates a voltage across the photodiode which causes the comparator inputs to switch polarity. Alternatively, when light from the LED ceases, the voltage across the photodiode collapses and the comparator inputs switch back to their initial state.
One of the critical limitations of optical isolators is the speed of the switching action. Switching speed may be impaired in a number of ways, one being by slow LED turn-on and turn-off. Since an LED is a current driven device, it requires a substantially square transition current waveform in order to have fast turn-on and turn-off times. A second factor that affects the speed of the optical isolator is the crossover time of the output comparator. A comparator has two reference inputs and an output, the output being dependent on the relative polarities of the reference inputs and the reference inputs being dependent on the photodiode voltage. The comparator output changes when the two reference inputs cross over and reverse polarity. The switching speed of the comparator is impaired where voltage potential between the reference inputs is large as it takes more time for the reference inputs to cross over.
It is, therefore, desirable to provide an optical isolator having improved turn-on and turn-off characteristics and enhanced switching reliability.